Documents listed below will be used as references for the following description.
Reference Document (1): S. Kubota, “Aplanatic condition required to reproduce jitter-free signals in optical disk system,” Appl. Opt. Vol. 26, pp. 3961–3973 (1987)
Reference Document (2): I. Ichimura, F. Maeda, K. Osato, K. Yamamoto, and Y. Kasami, “Optical disk recording using a GaN blue-violet laser diode,” Jpn. J. Appl. Phys. Vol. 39, pp. 937–942 (1999)
Reference Document (3): M. Itonaga, F. Ito, K. Matsuzaki, S. Chaen, K. Oishi, T. Ueno, and A. Nishizawa, “NA=0.85 single objective lens for a high density optical disk system,” Digest of International Symposium on Optical Memory, Taipei, pp. 26–27 (2001)
Reference Document (4): T. Ariyoshi, T. Shimano, and K. Maruyama, “0.85-NA single-objective lens using aberration-compensation methods,” Digest of International Symposium on Optical Memory, Taipei, pp. 268–269 (2001)
Reference Document (5): S. Ohtaki, N. Murao, M. Ogasawara, and M. Iwasaki, “The application of a liquid crystal panel for the 15 Gbyte optical disk systems,” Jpn. J. Appl. Phys. 38, pp. 1744–1749 (1999)
Reference Document (6): M. Iwasaki, M. Ogasawara, and S. Ohtaki, “A new liquid crystal panel for spherical aberration compensation,” Digest of Optical Data Storage Topical Meeting, SPIE 4342, pp. 103–105 (2001)
Reference Document (7): K. Osato, I. Ichimura, F. Maeda, K. Yamamoto, and Y. Kasami, “Progress in optical disk recording with over 20 GB of capacity,” Tech. Digest of Optical Data Storage Topical Meeting, Whistler, pp. 15–17 (2000)
Reference Document (8): T. Shimano, M. Umeda, and T. Ariyoshi, “Spherical aberration detection in the optical pickups for high-density digital versatile discs,” Jpn. J. Appl. Phys. 40, pp. 2292–2295 (2001)
For recording and playing back digital data, optical disk (including optical magnetic disks), such as CDs (Compact Disks), MDs (Mini-Disks), and DVDs (Digital Versatile Disks), used as recording media are known. Optical disk is a generic term for a recording medium in which a metal-film disk protected by plastic receives and reflects a laser beam and causes a change in the beam to produce a signal to be read out.
There are two types of optical disks, one of which is a playback-only type generally known as, for example, a CD, a CD-ROM, or a DVD-ROM, and the other one of which is a recordable type that is capable of recording user data generally known as, for example, an MD, a CD-R, a CD-RW, a DVD-R, a DVD-RW, a DVD+RW, or a DVD-RAM. In the recordable type, optical-magnetic recording, phase-change recording, and pigment-layer change recording, for example, are used to record data. The pigment-layer change recording is also known as write-once recording in which data is recordable only once and is not rewritable, and is thus suitable for use in data storage. On the other hand, optical-magnetic recording and phase-change recording are capable of rewriting data and are used for recording content data such as music, video, games, and application programs.
Furthermore, the recent development of a high-density optical disk called a DVR (Data & Video Recording) has dramatically increased the storage capacity.
In a disk drive, such as an optical recording/playback apparatus that performs recording and playback on these types of optical disks, when the spot size on the recording medium is indicated by φ, the wavelength of a laser beam is indicated by λ, and the numerical aperture of an objective lens is indicated by NA, the following equation (Equation (1)) is given:φ=λ/NA  Equation (1)
This implies that a shorter wavelength light source or a higher numerical aperture of the objective lens reduces the spot size φ, thus achieving high-density recording.
An objective lens used in an optical recording/playback apparatus is designed such that the lens has minimum wavefront aberrations with respect to a certain transmissive protection layer (cover layer) of the recording medium, such as a disk. In a CD drive, for example, the objective lens is optimized with respect to the 1.2 mm cover-layer thickness of the CD. Furthermore, in a DVD drive, the objective lens is optimized with respect to the 0.6 mm cover-layer thickness of the DVD.
In a DVD drive in which the information-recording layer of the DVD has a double-layer structure, an objective lens with a numerical aperture of 0.6 is used and a red laser diode having a wavelength of 650 nm is employed as the light source.
The tolerance of the objective lens with respect to different thicknesses of the cover layer is represented by the following equation (Equation (2)) from Reference Document (1):
                              W          40                =                              Δ            ⁢                                                  ⁢                          t              ⁡                              (                                                      n                    2                                    -                  1                                )                                      ⁢            N            ⁢                                                  ⁢                          A              4                                            8            ⁢                                                  ⁢                          n              3                                                          Equation        ⁢                                  ⁢                  (          2          )                    where Δt indicates the thickness variation of the cover layer, and n indicates the refractive index.
For example, if the value of tolerable spherical aberration (W40) is λ/4, the tolerable variation of the cover-layer thickness (Δt) of the DVD drive is ±27 μm. Regarding the double-layer disk used in the DVD drive, the distance between the two information-recording layers is predetermined at about 40 μm so as to fall within the tolerance mentioned above.
Reference Document (2) discloses a high-capacity optical-disk-drive achieved by providing a shorter wavelength for the light source and a higher numerical aperture for the objective lens. This optical disk drive uses a double lens having a numerical aperture of 0.85 and a blue-violet laser diode to achieve a DVD-size optical disk with a capacity of over 22 GB.
Recently, a single lens is used as an objective lens in place of the double lens and achieves a numerical aperture of 0.85. In comparison with the double lens, a larger working distance is obtained using the single lens. This technique has been disclosed in, for example, Reference Document (3) and Reference Document (4).
The optical disk drives described in Reference Documents (2) to (4) require a precise cover-layer thickness of not more than ±4 μm, derived from Equation (2).
In the optical disk drive of Reference Document (2) having the lens with a high numerical aperture, in order to achieve the same double-layer disk as in the DVD drive, about 20 μm of an interlayer distance is required to prevent interlayer interference of information signals. For this reason, the cover-layer thickness does not fall within the tolerance range (±4 μm).
Accordingly, Japanese Unexamined Patent Application Publication No. 2000-131603, for example, discloses a technique in which an expander lens is provided to compensate for spherical aberrations caused by the various thicknesses of the cover layer depending on the number of the information layers. Further effective compensation techniques that employ a liquid crystal element are disclosed in Reference Document (5) and Reference Document (6).
However, when the set amount of the spherical-aberration compensation employing the expander lens or the liquid crystal element is different from the required amount for the cover-layer thickness for the target information-recording layer, the spherical aberrations caused interfere with the generation of an optical focusing error signal. This may disadvantageously cause difficulties in the focusing operation.
In particular, when performing the focus control on an optical recording medium having two or more information-recording layers, the spherical-aberration compensation must be optimized for each target information-recording layer.
For this reason, there is a demand for an appropriate technique for focusing a laser beam to a focal spot so that a certain information-recording layer of a multilayer optical recording medium can be accessed using an objective lens having a high numerical aperture, for example, NA=0.85.
Furthermore, in an actual optical recording/playback apparatus, the case where a single-information-recording-layer recording medium is loaded and the case where a multiple-information-recording-layer recording medium is loaded may both exist. Consequently, for focus control, it is necessary to determine the number of information-recording layers in advance so as to perform the focusing operation on the desired information-recording layer.
In the case where the recording medium, such as a disk, is held in, for example, a cartridge, the number of information-recording layers may be determined by mechanically or optically detecting the presence of a detection hole provided in the cartridge for differentiating between disk types. However, if a disk is not held in a cartridge, an additional determining technique may be necessary.